Field of the Invention
The invention relates to agricultural machines, such as combine harvesters, having a header or similar attachment for harvesting or otherwise operating on a crop.
Description of the Prior Art
Such machines are known to have automatic height adjusting means for automatically adjusting the position of the header above the ground. One form of known automatic header height control means for combines comprises one or more ground sensing members pivotally connected to the header and arranged to control a hydraulic lifting mechanism for the header. This is to say that the ground sensing members detect elevations and depressions in the terrain and control the supply of fluid to or the discharge of fluid from the header lifting mechanism accordingly. Therefore, the ground sensing members are coupled in some way to a hydraulic valve included in the hydraulic header lifting mechanism. This type of header height control mechanism can operate satisfactorily and relieves the operator from having continuously to check and readjust the position of the header relative to the ground. However, this type of automatic header height control mechanism has a number of disadvantages of which an important one resides in the fact that such mechanisms have a rather complicated structure and are therefore relatively expensive to manufacture. Also, it is desirable to sense the ground contours at a location in front of the header cutter mechanism which can best be accomplished by providing the ground sensing members underneath the normal crop dividers which extend at the side edges of the header and in front thereof. However, as a result, any terrain variations located between the side edges of the header are not detected.
According to other approaches made in the past, less complicated and hence less expensive automatic header height adjusting mechanisms comprising header weight-compensation means have been proposed. These types of adjusting mechanisms are generally known as header compensation mechanisms and resilient means are provided to support a portion of the header weight. The resilient means may be of any suitable type such as the mechanical type comprising tension springs or compensation springs, or of the hydro-pneumatic type comprising a hydro-pneumatic accumulator. In operation, the header is lowered until it contacts the ground under a certain pressure, whereby a portion of the weight of the header is supported on the ground. In this normal header operating position, an equilibrium is set up in which a major portion of the header weight is supported by the weight-compensation mechanism and the header pivot means to the basic unit and in which the remainder of the weight is supported on the ground as already indicated. If, during forward movement of the machine in the field, a ground irregularity is met, e.g. an elevation, then the equilibrium referred to is disturbed as an increased portion of the header weight is then supported on the ground. The weight-compensation mechanism is operative to reinstate equilibrium by raising the header in accordance with the magnitude of the elevation causing the disturbance. If, on the contrary, a depression in the ground is met, then the header weight-compensation mechanism is operative to readjust the position of the header in the reverse direction. It is thus seen that the header is arranged to float over the ground irrespective of ground irregularities.
In general, the header weight-compensation mechanism at present known in the art operate in an appropriate manner provided the initial ground pressure of the header is accurately set. Header weight-compensation mechanisms have, as mentioned already, the advantage of being less complicated, and hence less expensive, than other systems. Nevertheless such mechanisms all have an important common disadvantage.
To function appropriately it is an absolute requirement for the operator to pre-set the header in an optimum position. This means that the initial ground pressure of the header should neither be too high nor too low. If the ground pressure is too high, then the resilient weight-compensation means has to expand to an extent that they are no longer able sufficiently to raise the header upon meeting an elevation in the ground. As a result, the header tends to dig in the ground. If, however on the contrary, the initial ground pressure is too low, then it might happen that during operation the header temporarily loses contacts with the ground.
In practice, this pre-setting of the header has proved to be very critical and difficult to achieve. Furthermore, it has been experienced that continuous attention should be given to the operation of the header weight-compensation means because from time to time readjustment of the pre-set position of the header is required. Also, it should be kept in mind that the hydraulic header lifting mechanism is arranged to raise and lower the header comparatively fast. This lack of fine control aggravates the problem of pre-setting the header in the operative condition.
In still another approach to the problem, an automatic header weight-compensation mechanism has been provided comprising pressure-responsive components in the hydraulic circuitry operable to shut off fluid discharge from the hydraulic lifting mechanism when the pressure on a hydro-pneumatic accumulator reaches e.g. 85% of the maximum pressure (this means 85% of the pressure when the header is fully raised). In other words, in this arrangement about 85% of the weight of the header is supported by the hydraulic lifting mechanism while the remainder of the weight is supported on the ground. The operation is as follows: assuming that the system is in its "automatic" position and assuming that initially the header is fully raised, then the pressure in the lifting mechanism and the hydro-pneumatic accumulator is at its maximum. As a result, a pressure-responsive relief valve, which is pre-set at e.g. 85% of the maximum pressure, opens and hydraulic fluid can drain from the hydraulic lifting mechanism and the accumulator, whereby the header is lowered. This lowering continues until 15% of the header weight is supported directly on the ground, whereupon the relief valve shuts off further discharge of hydraulic fluid.
An important disadvantage of this arrangement resides in the fact that the shutting off operation is not initiated unless the header touches the ground, and as a result of the inertia of the system, the minimum ground pressure of the header cannot be held below a certain value as is sometimes absolutely necessary to ensure adequate operation of the system without the tendency of the header to dig into the ground.
In still a further approach, which is to some extent similar to that just described, a pressure-responsive relief valve is included in the hydraulic circuitry and is operable to shut off the fluid supply to the hydraulic lifting mechanism and the hydro-pneumatic accumulator (rather than the fluid discharge therefrom) as soon as a predetermined pressure is reached. Simultaneously, fluid discharge from the lifting mechanism and the accumulator is also prevented. From that moment onwards, the pressure line of the hydraulic pump is connected to the return line via the relief valve. This is also disadvantageous as the relief valve, which is operative most of the time, will heat the hydraulic fluid and unnecessarily consume a considerable amount of horsepower.
Furthermore, all systems employing pressure-responsive relief valves have the common disadvantage of only enabling slow movement of the header, particularly when the actual pressure is close to the opening pressure of the relief valve. It is the principal object of this invention to overcome or attenuate these disadvantages.